Hydroxyapatite (chemical formula: Ca10(PO4)6(OH)2) is the main inorganic phase of human body hard tissue, such as bone and teeth, with good bioactivity and biocompatibility. The hydroxyapatite coatings deposited on the biomedical metal substrates by air plasma spraying are ideal hard tissue replacement and repair materials, which combine the excellent mechanical properties of biomedical metal with the good bioactivity and biocompatibility of hydroxyapatite. Hydroxyapatite coatings prepared on the biomedical metal substrates have been used extensively and successfully in commercial applications such as dental implants, artificial joints and other medical implants. The principle of air plasma spraying is as follows: firstly, the hydroxyapatite powders are heated to molten state in a very short time using 5000-10000° C. high temperature plasma flame; then, the molten hydroxyapatite is propelled to the surface of biomedical metal at a high speed of 300 m/s, to form strong adhesion between the hydroxyapatite coating and the biomedical metal. However, the hydroxyapatite coating deposited on the implant surface is usually dense and the porosity is small, and a scanning electron microscopy (SEM) micrograph is shown in FIG. 1. After the implants are put into the defect site, it is difficult to grow the new tissue into the implant surface, thus affecting the early fixation and long-term stability of the implants.
The preparation of porous hydroxyapatite coatings is the best way to solve the above problem caused by the dense implant surface. However, due to the restriction of the specific principle of the plasma spraying, it is very difficult to prepare the porous hydroxyapatite coatings using air plasma spraying and to get the hydroxyapatite coatings with the good porous structure. The porous titanium coatings have been deposited on the implant surfaces by air plasma spraying. The porous titanium coatings not only have a good three-dimensional porous structure, but also possess good mechanical properties. However, owing to its bio-inertness, it is usually in need of subsequent biological activation to improve the bioactivity, such as acid treatment, alkali treatment, acid-alkali treatment, and anodic oxidation. Usually, these treatments often need very long process times, and the bioactivity of the porous titanium coatings treated by alkali treatment is less than that of the porous calcium phosphate materials, thus needing a longer time to form new bone. (Takemoto M, Fujibayashi S, Neo M, Suzuki J, Matsushita T, Kokubo T, Nakamura T. Osteoinductive porous titanium implants: Effect of sodium removal by dilute HCl treatment. Biomaterials, 2006, 27(13):2682-2691.)
The previous studies of the inventors (CN101250681 and CN101250682) provide a method and equipment for preparing bioactive hydroxyapatite coatings through the suspension plasma spraying process. The method selects the liquid precursor plasma spraying process, instead of the air plasma spraying process. In the process, the hydroxyapatite suspension is transferred into the suspension injection system by peristaltic pump. Then, the hydroxyapatite suspension is injected into the high temperature area of the plasma flame, where the suspension makes the heat exchange with the high temperature plasma flame from the plasma spraying gun. Then the suspension is subjected to breakup and refinement of the droplets, solvent evaporation, and melting of spraying raw materials. Finally the bioactive hydroxyapatite coatings with a uniform structure are directly deposited on the substrate surfaces of the biomedical metal materials. Moreover, in the suspension plasma spraying preparation for making porous hydroxyapatite coatings, it directly selects the spraying feedstock materials prepared through wet chemical method, which avoids the complicated preparation processes for APS feedstock materials, such as aging after wet chemical synthesis, spraying drying, powder granulation and spheroidization, calcination, ball milling and sieving. It shortens the total process time and cuts the process costs.